Reading: Chapter 3 – Bowman et al. p 29 – 59. Head-Mounted Display Reading: Chapter 3 – Bowman et al. p 29 – 59. Hodges and Babu 2008

Visually Coupled Systems A system that integrates the natural visual and motor skills of an operator into the system he is controlling. Basic Components An immersive visual display (HMD, large screen projection (CAVE), dome projection) A means of tracking head and/or eye motion A source of visual information that is dependent on the user's head/eye motion. Hodges and Babu 2008

Head-Mounted Displays Optical System Image Source (CRT or Flat Panel (LCD)) See–Through or Non–See–Through Mounting Apparatus Earphones Tracker (Pos & Ori) Hodges and Babu 2008

Field of View Monocular FOV is the angular subtense (usually expressed in degrees) of the displayed image as measured from the pupil of one eye. Total FOV is the total angular size of the displayed image visible to both eyes. Binocular(or stereoscopic) FOV refers to the part of the displayed image visible to both eyes at the same time. FOV may be measured horizontally, vertically or diagonally. Hodges and Babu 2008

Field of View (FoV) vs. Field of Regard (FoR) A display’s FoR refers to the amount of physical space surrounding the user in which visual images are displayed. FoV refers to the maximum number of degrees of visual angle that can be seen instantaneously on a display. For Instance; In a HMD, the user may have 50 degree horizontal FoV, but 360 degree FoR. Hodges and Babu 2008

Focal Length & Diopter Focal Length - The distance from the surface of a lens (or mirror) at which rays of light converge. Diopter - The power of a lens is measured in diopters, where the number of diopters is equal to 1/(focal length of the lens measured in meters). Hodges and Babu 2008

Ocularity and IPD Ocularity Interpupillary Distance (IPD) Monocular - HMD image goes to only one eye. Biocular - Same HMD image to both eyes. Binocular (stereoscopic) - Different but matched images to each eye. Interpupillary Distance (IPD) IPD is the horizontal distance between a user's eyes. IPD is the distance between the two optical axes in a binocular view system. Hodges and Babu 2008

Vignetting and Eye Relief The blocking or redirecting of light rays as they pass through the optical system. Eye Relief Distance Distance from the last optical surface in the HMD optical system to the front surface of the eye. Hodges and Babu 2008

Basic Eye Cornea Crystalline Lens Fovea Optic Nerve Retina Hodges and Babu 2008

The Eye Accommodation - Term used to describe the altering of the curvature of the crystalline lens by means of the ciliary muscles. Expressed in diopters. Retina - The sensory membrane that lines the back of the eye and receives the image formed by the lens of the eye. Fovea - The part of the human retina that possesses the best spatial resolution or visual acuity. Hodges and Babu 2008

Properties of the Eye Approximate Field of View Acuity 120 degrees vertical 150 degrees horizontal (one eye) 200 degrees horizontal (both eyes) Acuity 30 cycles per degree (20/20 Snellen acuity). Hodges and Babu 2008

Simple Formulas Visual Resolution in Cycles per degree (Vres) = Number of pixels /2(FoV in degrees) Example: (1024 pixels per line)/(2*40 degrees) = Horizontal resolution of 12.8 cycles per degree To convert to Snellen acuity (as in 20/x) Vres = 600/x (20/47) Hodges and Babu 2008

Optical System Move image to a distance that can be easily accommodated by the eye. Magnify the image Hodges and Babu 2008

Simple Magnifier HMD Design q p f Image Eye Eyepiece (one or more lenses) Display (Image Source) Hodges and Babu 2008

Thin Lens Equation 1/p + 1/q = 1/f where p = object distance (distance from image source to eyepiece) q = image distance (distance of image from the lens) f = focal length of the lens Conventions: If the incident light comes from the object, we say it is a real object, and define the distance from the lens to it as positive. Otherwise, it is virtual and the distance is negative. If the emergent light goes toward the image, we say it is a real image, and define the distance from the lens to it as positive. f = positive for a converging lens A light ray through the center of the lens is undeflected. Hodges and Babu 2008

Virtual Image Virtual Image Lens Display Hodges and Babu 2008

LEEP Optics Large Expanse Extra Perspective Give very wide field of view for stereoscopic images Higher resolution (more pixels) in the middle of the field of view, lower resolution on the periphery Pincushion distortion Hodges and Babu 2008

Fresnel Lens A lens that has a surface consisting of a concentric series of simple lens sections so that a thin lens with a short focal length and large diameter is possible More even resolution distribution Less distortion Hodges and Babu 2008

Distortion in LEEP Optics A rectangle Maps to this Hodges and Babu 2008

To correct for distortion Must predistort image This is a pixel-based distortion Graphics rendering uses linear interpolation! Too slow on most systems Hodges and Babu 2008

Distorted Field of View Your computational model (computer graphics) assumes some field of view. Scan converter may over or underscan, not all of your graphics image may appear on the screen. Are the display screens aligned perpendicular to your optical axis? Hodges and Babu 2008

Distorted FoV (cont.) Distance along z-axis Hodges and Babu 2008

Collimated: p=f 1/p + 1/q = 1/f q = , if p=f If the image source is placed at the focal point of the lens, then the virtual image appears at optical infinity. f Hodges and Babu 2008

Compound Microscope HMD Design Relay lens produces a real image of the display image source (screen) at some intermediate location in the optical train. The eyepiece is then used to produce an observable virtual image of this intermediate image. Relay Lens Image Intermediate Real Image Eyepiece Exit Pupil Hodges and Babu 2008

Exit Pupil The area in back of the optics from which the entire image can be seen. Important if IPD not adjustable, mount not secure. Compound microscope optical systems have a real exit pupil. Simple magnifier optical systems do not have an exit pupil. Hodges and Babu 2008

Characteristics of HMDs Immersive You are inside the computer world Can interact with real world (mouse, keyboard, people) Ergonomics Resolution and field of view Tethered Hodges and Babu 2008

In Class Assignment Go to http://www.virtualresearch.com/ Figure out for the VR1280 HMD Vertical and Horizontal Resolution Vertical and Horizontal FoV Vertical and Horizontal Equivalent Snellen acuity Hodges and Babu 2008

OSG Tips Loading objects and adding to scene hierarchy: osg::Node *bicycle_node = (osg::Group*)osgDB::readNodeFile("bike.obj"); bicycle_node->setName("bike"); osg::PositionAttitudeTransform* bicyclePAT = new osg::PositionAttitudeTransform; bicyclePAT->addChild(bicycle_node); bicyclePAT->setPosition(osg::Vec3(0.2, 0.0, 0.38)); //Position bicyclePAT->setScale(osg::Vec3(0.0148, 0.0148, 0.0148));//0.015 //Scaling bicycle_shadow = (osg::Group*)osgDB::readNodeFile("C:/bike_shadow.obj"); bicycle_shadow->setName("bike_shadow"); bicyclePAT->addChild(bicycle_shadow); //Adding Shadow to the Bicycle object osg::Quat quat = bicyclePAT->getAttitude(); //set rotation of the bicycle quat.makeRotate(90.0*3.1415926/180.0,osg::Matrixf::value_type(0.0f),osg::Matrixf::value_type(0.0f),osg::Matrixf::value_type(1.0f)); bicyclePAT->setAttitude(quat); Hodges and Babu 2008

OSG Tips Continued… Loading objects and adding to scene hierarchy: osg::Group *root; root->addChild(bicyclePAT); viewer.setSceneData(root); Hodges and Babu 2008

OSG Lights Hodges and Babu 2008 // Lighting code osg::ref_ptr<osg::Group> lightGroup (new osg::Group); osg::ref_ptr<osg::LightSource> lightSource1 = new osg::LightSource; //light pos osg::Vec4f lightPos1 (osg::Vec4f(-2.3147,-0.3335,1.6764,1.0f)); //light parameters osg::ref_ptr<osg::Light> myLight1 = new osg::Light; myLight1->setLightNum(1); myLight1->setPosition(lightPos1); myLight1->setAmbient(osg::Vec4(0.5f,.5f,.5f,1.0f)); myLight1->setDiffuse(osg::Vec4(.5f,.5f,.5f,1.0f)); myLight1->setConstantAttenuation(1.0f); lightSource1->setLight(myLight1.get()); lightSource1->setLocalStateSetModes(osg::StateAttribute::ON); lightSource1->setStateSetModes(*lightSS,osg::StateAttribute::ON); //add to scene graph lightGroup->addChild(lightSource1.get()); root->addChild(lightGroup.get()); Hodges and Babu 2008